The rod shaped structures in the nucleus of a cell are chromosomes. They carry genetic material in a "rod" of DNA that is bonded to various proteins in the cell nucleus.
The word "chromosome" is derived from the Greek words "chroma," meaning "color," and "soma," meaning "body." Chromosomes condense into thicker structures and align on metaphase plates while mitosis occurs.
Humans normally have 23 chromosome pairs. Chromosomes in most somatic cells occur in pairs because one member of a pair comes from the mother and the other chromosome comes from the father.
In viruses, chromosomes may appear as short circular or linear structures that contain the DNA or RNA molecules.